1. Field of the Invention
This invention relates to antenna receiving apparatus for receiving radio frequency signals and, more particularly, to antenna feed apparatus for receiving polarized radio signals.
2. Description of the Prior Art
Satellite to ground transmission has become increasingly popular over the last twenty years. It offers the advantage of a single ground to satellite uplink point of being able to transmit its information to virtually every part of the United States or to a majority of a hemisphere of the earth.
One of the most popular uses of satellite to ground transmissions is for TeleVision Receive Only or TVRO. It is used by TV networks to get their messages to as many local affiliates and into as many households as possible. Within the last ten years approximately three million home owners in the United States alone have invested in their own TVRO or satellite dish receiving system.
Since the signal transmitted from a satellite is generally very weak, it is important that the signal, which is gathered by an antenna and focused into a feed, is amplified by a low noise amplifier (LNA) which contributes as little amplifier noise to the signal as possible. Since all LNAs add noise to the signal which is being amplified, it is important that the LNAs are designed to minimize the amplifier noise contribution. It is also important that the polarization of the feed is aligned with the incoming signal to maximize the signal to noise ratio. Since the amplifier will add a fixed level of noise power to the signal even when the polarization of the feed is misaligned, the signal to noise ratio of the feed system will be less than maximum when the polarization of the feed is not properly aligned.
Signals are typically transmitted from a satellite in either linear or circular polarizations. In order to maximize the use of a satellite bandwidth, most satellites will transmit two sets of signals within the same frequency band but at polarizations which are orthogonal to each other. For example, a satellite can transmit one set of signals with a vertical linear polarization and a second set of signals with a horizontal linear polarization, or a first set with right hand circular polarization (RHCP) and a second set with left hand circular polarization (LHCP).
The receiving ground system must have a feed which can adapt to the transmitted polarizations. This practice of transmitting and receiving two orthogonal polarizations is commonly referred to as frequency reuse.
Although the cost of home satellite dish systems has dramatically dropped in the last ten years, the cost of such a system still prevents many homes from investing in a satellite dish. The present invention is a simple and low cost antenna feed apparatus for receiving polarized signals from satellites, and, at the same time, gives increased performance over the feed apparatus in common use at the present time.
The most commonly used feed for the home satellite dish market is an electro-mechanical design which was made popular by Chaparral Communications Corp. of San Jose, Calif. This design, which is described in U.S. Pat. No. Re. 32,835 (Howard), has a probe disposed through the center of the back wall of a circular waveguide. An electric servo-motor is used to rotate the probe to pre-set positions where it is aligned to the electric field of the incoming satellite signal.
Since the '835 apparatus will adapt to any sense of linear polarization, it can be mechanically installed without regard to polarization alignment. The apparatus of the '835 patent may be contrasted to fixed polarization designs which often require repeated mechanical adjustments and tests during the installation process.
After installing the type of feed shown in the '835 patent, the homeowner, through the use of a polarization control circuit, is able to determine the preset positions of the probe. The apparatus of the '835 patent, which is limited to receiving only linear polarizations, has inherent losses which reduce the signal to noise ratio. Since the probe must be mechanically rotated each time the operator desires to switch frequency reuse bands, its response time is noticeably slow. This type of feed is normally connected directly to a Low Noise Amplifier (LNA) or a Low Noise Block Downconverter (LNB) to complete the antenna feed circuitry.
An all electronic feed which makes use of two orthogonal signal paths to form an adaptive polarization system is described in U.S. Pat. No. 5,068,668 (Tsuda et al). The '668 patent describes circuits which are very complex and which are not likely to instantaneously perform well over the total bandwidth of a TVRO system. The apparatus of the '668 patent does not make use of variable attenuators or variable gain amplifiers in each of the two signal paths which could have simplified his circuits. In the presence of two incoming frequency reuse signals with orthogonal polarizations, the Tsuda et al circuit is likely to become confused about which polarization to lock onto. Furthermore, the circuit provides no means for switching between the two orthogonal polarizations.
In U.S. Pat. No. 4,233,576 (Pelchat) describes an adaptive polarization receiver system which is a complex arrangement of waveguide polarizers, OMT's, and phase sections. This type of system is probably too expensive and is probably too cumbersome to be of practical use in the home satellite dish market.